1. Field of the Invention
This invention relates to a panel-shaped loudspeaker comprising a panel and at least an exciter coupled to the panel, said panel being provided with two, at least substantially parallel, interconnected walls.
2. Description of The related Art
A loudspeaker of this type is known from PCT Patent Application WO-A 97/09842.
The known panel-shaped loudspeaker comprises a panel having a sandwich-like structure and a rigid cellular core such as a honeycomb structure, and two skins glued to the core, the core extends extending the skins. A light metal and a synthetic material are used as materials for the core. Paper, cardboard, synthetic material and light metal are mentioned as materials for the skins. The omni-sided relatively rigid lightweight panel exhibits usable patterns of bending waves only at relatively high frequencies. In the known loudspeaker, one or more exciters are used which are arranged with respect to the panel in such a way that bending waves are produced only at relatively high frequencies. Due to reflections on the edges of the panel, bending waves which interfere with each other are spread across the panel. The known loudspeaker has a poor acoustical behavior in the low-frequency range.
It is an object of the invention to provide a panel-shaped loudspeaker which has a satisfactory acoustical behavior, both at relatively low frequencies and at higher frequencies.
This object is achieved with the loudspeaker according to the invention, which is characterized in that the panel comprises a structure of strip-shaped partitions extending between the walls of the panel, the longitudinal axes of all of said partitions mutually extending at least parallel to each other and parallel to the walls, said partitions being further secured to the walls, the walls and the partitions being made of a material which, used in the panel, has an internal damping which is at least 2.5% of the critical damping of the relevant material, used in the panel. Critical damping is herein understood to mean that damping at which the oscillating character of the panel stops.
Mechanically, the panel of the loudspeaker according to the invention is anisotropic, the panel being bendable around an axis extending parallel to said longitudinal axes and being relatively bending-stiff about an axis oriented transversely thereto. Surprisingly, it has been found that the loudspeaker according to the invention has a favorable acoustical behavior, both at low frequencies, central frequencies and high frequencies. It has been found, by experiments, that natural resonances already occur at relatively low frequencies in the panel of the loudspeaker according to the invention, these resonances having a smaller amplitude only at natural resonances occurring at relatively high frequencies, as compared with the loudspeaker known from said WO-A 97/09842. It is preferable not to exceed an internal damping of maximally 10% so as to avoid negative effects possibly occurring with larger dampings at higher frequencies. A very favorable acoustical behavior through a wide frequency range was found at an internal damping of 3% of the critical damping.
An embodiment of the loudspeaker according to the invention is characterized in that the partitions mutually extend at least substantially parallel to each other and are at least substantially perpendicular to the walls for achieving an optimal anisotropy.
A practical embodiment of the loudspeaker according to the invention is characterized in that the material of the walls is equal to the material of the partitions. A further embodiment is characterized in that the walls and partitions blend seamlessly. In this embodiment, the walls and partitions constitute an integral unit manufactured, for example, by means of extrusion. Such a panel is obtainable in a simple manner and at low cost.
An embodiment of the loudspeaker according to the invention is characterized in that the material of the walls and the partitions of the panel is a polypropylene. The polypropylene is preferably a co-polymer.
An extruded, twin-walled plate of polypropylene co-polymer suitable as a loudspeaker panel is commercially available. Measurements proved that this material had an internal damping of 2.9% of the critical damping factor of the relevant material.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.